linux-kernel - Re: vfs: INFO: possible circular locking dependency detected

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Message-ID: <20120509161203.GK22082@ZenIV.linux.org.uk>
Date:	Wed, 9 May 2012 17:12:03 +0100
From:	Al Viro <viro@...IV.linux.org.uk>
To:	Sasha Levin <levinsasha928@...il.com>
Cc:	linux-fsdevel@...r.kernel.org,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: Re: vfs: INFO: possible circular locking dependency detected

On Wed, May 09, 2012 at 05:25:14PM +0200, Sasha Levin wrote:
> Hi all,
> 
> I've started seeing the following warning while fuzzing inside a KVM guest with the latest -next:

[->read() may grab ->cred_guard_mutex, but it may itself be called by
prepare_binprm() after having ->cred_guard_mutex grabbed]

Nasty, that...  What's more, it's not just prepare_binprm() itself -
->load_binary() might end up calling read(); it doesn't have to
limit itself to mmap(), so essentially anything that can be grabbed
by ->read() of a regular file might nest under ->cred_guard_mutex.

	AFAICS, /proc/*/stack, /proc/*/syscall, /proc/*/personality,
/proc/*/io_accounting, /proc/*/auxv, /proc/*/environ, /proc/*/*maps
and /proc/*/pagemap have ->cred_guard_mutex grabbed on read.  seq_file
is a red herring here - io_accounting has the same issue and it does
things directly, without seq_read().

	It's not a realistic attack, fortunately, since you need root
to get past open_exec() on any of those...  Wait.  How _did_ you get
past open_exec(), anyway?  MAY_EXEC is not supposed to be granted on
anything that has no exec bits at all and AFAICS none of those files
have them.
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